Useful low orbital shells around the Earth are an awful lot smaller. They're still big, like the sky is big, but airplanes do manage to, at least somewhat regularly, collide with each other "in the middle of nowhere" (most midairs are around airports, since there are a lot of airplanes heading in and out of them).
Aircraft debris, however, falls out of the sky in short order. Orbital debris doesn't.
The probability of an initial collision is fairly low, but once you have a few, or someone decides to show off their new ASAT weaponry (which is functionally a satellite collision, just one of them only having been in space for a short while), you now have flak that won't go away, moving in all directions at orbital velocity. That, then, has the potential to start causing all sorts of problems, and once that's taken out a few more satellites with orbital-energy impacts, you start having a real problem.
Will it happen? Those invested in littering certain useful orbital shells with satellites claim not. We'll see.
I've heard SpaceX claims to deal with it by putting their satellites in an orbit that requires periodic boosting to stay put -- if they don't, satellites decay and de-orbit after a few years.
The plan is, I think, to regularly retire (de-orbit) and replace these satellites so they can continuously improve the design, and as a nice side-effect any collisions or otherwise misbehaving objects will at least stop being a problem after a known maximum span of time.
Now, how many other satellites are likely to get hit in "5-10 years" of flak flying around at orbital velocities? It's nice if your goal is to say "Well, it probably won't render space useless forever..." - but that assumes one collision, and no ongoing collisions. If it's "5-10 years per collision," and each new one resets the 5-10 years timeframe, well, there's a lot of stuff up there that can make a mess.
Unfortunately, the low orbital shells are also "everywhere." It's not like geosync orbits where if you Kessler them, well... not going to have geosync stuff anymore, but you can still use other orbits. If you Kessler LEO shells, you pretty much have a high risk of anything you try to send to space getting blasted on the way up. It doesn't take much of a bit of debris, at orbital speeds, to trash your booster and make you have a bad day. I guarantee a bolt at 17,000 mph will do some damage (typical LEO velocities) - that's about ~22x the speed of sound in the atmosphere, depending on altitude.
If there are tens of thousands of things up there and you start things colliding and blowing up, you've basically put a defensive shell around the earth for a century, if not longer, by the time all the satellites get fragged and the debris eventually comes down.
It's not.
The concern is that unwanted collisions will become common enough that satellites break down or are completely destroyed within months. There will never be enough debris that we can't fly through LEO.
Flying through and flying through safely are very different things. With tens of thousands of objects in and below LEO and (presumably) increasing flights to orbit, it's only a matter of time until a rocket gets taken down. It doesn't require a particularly large piece for an involuntary update to your deorbiting plans.
> There are currently over 1,600 Starlink satellites in orbit, and that number will continue to grow; SpaceX has filed paperwork for up to 42,000 satellites for the constellation.
https://www.space.com/spacex-starlink-internet-satellites-la...
They're aware of the risk of collisions, but toss in a few more vendors trying for the same size, and you can have potentially hundreds of thousands of satellites in the shells. They're big, but they're not that big.
Obviously that is something to avoid, but it's not like we're filling up a non-renewable resource in low earth orbit. It cleans itself naturally.
FWIW.
(RIP Douglas Adams)